Doing more CPU intensive things like playing a DVD or a 3D video game drains more battery power. Doing simple word processing with screen on low brightness may last a very long time.
Btw, do you own a Mac? if so, you might want to calibrate your battery every month or so.
Detailed instructions are here: http://support.apple.com/kb/ht1490
Basically you have to make full charge cycles for a Lithium-Ion to stay in “good shape”.
You’ve got to charge till 100% and leave it like that for min.2 hours and then drain the whole battery till sleep-mode and then leave it on that reserverve power for min.5 hours till the sleep light goes out.
Then charge it up again (without use) till 100%.
That way the battery “knows” what its extreme low and high ranges of charge are and you keep all cells in the battery active.
Guess this works for every portable device with a Li-Ion battery.